The Study Of 4H-SiC Ultraviolet Phototransistor Based On BJT Structure

As the third-generation semiconductor material,4H-SiC material has attracted wide attention because of its wide band gap,high breakdown electric field,high thermal conductivity,high electron saturation rate,and radiation resistance,etc.Compared with the traditional Si-based UV detector,the UV detector made by 4H-SiC material has a great research value in detecting the ultraviolet signal due to the advantage of shielding the background of visible and infrared light.At present,the research of SiC-based UV detectors mainly focuses on P-i-N photodiodes,MSM detectors and APD avalanche diodes.Compared with the above three detectors,BJT-structured SiC UV detectors have higher gain,lower bias voltage and small internal noise.In this paper,the working principle,device structure design,static and dynamic characteristics simulation and fabrication process and measurement of BJT structure UV phototransistors have been studied in detail.The main research methods and contents are as follows:Firstly,different structural parameters of BJT-structured UV phototransistors are studied:The doping concentration and thickness of different regions are simulated by ISE TCAD device simulation software.By analyzing the spectral response of the same parameters of the device,the spectral response range of the device is 200 nm.-380 nm,which belongs to the ultraviolet region,reaches a peak at a wavelength of 270 nm.By analyzing the influence of doping concentration and thickness on the current gain in different regions,the optimal structural parameters of the device are obtained:the thickness of the n type collector region is 0.2 ?m,doping concentration is 1×1019 cm-3,the thickness of the p-type base region is 1.0 ?m,doping concentration is 3×1019 cm-3,the thickness of the n+type emitter region is 2.0 ?m,doping concentration is 1×1019 cm-3.Secondly,the transient response of BJT-structured UV phototransistors under different light intensities are studied:the response time of the device under different light intensities is obtained.At a light intensity of 1×10-4 W/cm2,the response time of the device is in the order of ms.As the light intensity increases by the order of magnitude,the response time decreases by the order of magnitude.The delay time of the device is longer than the rise time,while the long charging time of the emitter junction is the main reason by analyzing internal mechanism.Comparing the static and transient characteristics between phototransistor and photodiode,at 270 nm,the optical responsivity of phototransistor is 65 A/W compared to the photoreceptor 's optical responsivity of 0.5 A/W.However,due to the above-mentioned effects of the charging capacitance of the emitter junction,the transient response time of the phototransistor is much larger than that of the photodiode.Finally,the fabrication process and layout of the BJT structure of the UV phototransistor are completed.First of all,the epitaxial wafer material is prepared and characterized,and then on the front side and the back side of the device,metal is deposited separately by used Ni material.For the ohmic contact process,the specific contact resistance is on the order of 10-5 ?·cm2.Through the I-V characteristic test of the device,the light dark current ratio of the device is compared in three orders of magnitude.Based on the previous experiment,an improved process and the layout with isolation are also proposed.Through the second experiment,The optical responsivity of the device reached 0.1 A/W at 254 nm.and reached 5.4×10-4 A/W at 365 nm,and the light-dark current ratio reaches five orders of magnitude.The linearity is better under different optical powers,and the response time of the device is faster,the rise time is 200 ms,and the fall time is 80 ms.Finally,the same device in different units has good consistency.